Delta:wye xfrmr cut out fuse blown 1

[Dyorsolutions]

I have multiple transformers (1000kv, 2300kv, 3200kv)

Each are 3phase 415/240v

Each transformer powers multiple units. When I does amperage readings on each phase of the 400amp breakers I get various results such as:

91a/175a/235a

Could uneven phases like this be the culprit of making its way back up the power pole to blow one of the cutout fuses?

Thank you.

 

[RRaghunath]

The currents measured indicate that the three phases are not loaded same. But, these are still load currents only and cannot cause fuse to blow on HV side of transformers.
Fuse blow can happen only if the current through the fuse is more than the fuse Ampere rating.
It is possible that one or two phases are overloaded on primary side of transformers due to too much unbalance in loading of 415/240V system.
You may want to check.

 

[waross]

Transformer KVA ratings?
Primary fuse ratings?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dyorsolutions]

@waross

3200 kva xfrmr

HV 12470 D (we have on tap E, as utility wasnt bringing in voltage for what we needed at “d”).

E tap is - 10800v 171.1a

Cutout fuse is 140amp

With partial load we’ve been okay, but full load blows the fuse on HV riser every time. We’ve done that 4 times in past week.

 

[Kiribanda]

1) Where is your 400A bkr located? Is it on the 12.47kV high side?
2) If yes, in addition to this 400A bkr, are there 12.47kV pole fuses at the service point which you say blowing?
3) What is the nominal system voltage of the Utility side which you have signed the contract? Is it 12.47kV or 10.8kV?
4) What will happen if Utility feeding your transformer at 12.47kV while your operating tap is 10.8kV?

 

[Dyorsolutions]

@kiribanda

Great questions and kind of the same path I’ve been walking down:

400a breakers in unit (building), 3 breakers in each panel (this particular xfrmr supplies 4 units (buildings) so (12) 400a breakers being supplied by the xfrmr secondary (3ph w/ neutral).


On HV lines rated 12470kv we have 3 primary wires coming down via individual conduit; each going into phase of xfrmr (primary side). Each primary line (4/0 awg, 15kv, 133% insul 220mils) has a cutout fuse located at top of pole on riser; each rated at 150amps (though the same phase cutout keeps blowing each time) at 12470kv the max amps on xfrmr is rated 148.2a (xfrmr placard in photo on my initial post)

Signed contract is for 12470kv. However at this contract service rating when we energized our secondary was reading low voltage (approx. 388/218v). So we tapped down to “E” (10800kv with 171.1amp max)which brought us within range (approx. 418/245v).

So if one of our phases with 4 units running was maxing out the amperage demand it could exceed the 150a cutout fuse on top of pole riser & blow it.


Was thinking of having utility:

-test the line voltage
-test each primary leg amperage
-refuse cutouts on riser with a 175a or 200a fuse (seeing as our 4/0 primary wire is rated up to 238amps)
-then introduce full load again


Our current setup can for sure run 2/4 units
Not sure about 3/4 units
But certainly blows the cutout fuse if 4/4 units running; usually within an hour.

 

[waross]

You are using 140 Amp fuse and blowing at full load.
The transformer nameplate shows full load amps of 171.1 Amps at full load on E tap.
The fuses may be doing exactly what they are supposed to do.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dyorsolutions]

@waross

That’s where I was concluding at as well after running through the facts and think out loud here.

Let me throw a variable in here to get your thoughts on.

A “full load” for us is normally 80% of the transformers max.
Also we have 2 other transformers of the same specs at this work site that have 100amp fuses at top of pole and neither of those have had any issues on its “full load”.


(So extra details: the 3phase current comes off secondary and into electrical panel at 415/240 range and then arrives at power source that way, it then hits a power receptacle that has an internal bus bar that delivers 240v single phase to each receptacle ::4 receptacle outlets per phase:: so it creates a difficult task to perfectly balance out what amperage is being demanded from each phase, causing an extremely unbalanced load at the breaker such as 230a/130a/90a legs)

Our utility company only had a 200a cutout fuse in truck so they replaced the fuse that keeps blowing with that 200a fuse but the rest of the fuses on all risers are 100amp and doing ok, so they’re telling me it seems to be a transformer going bad (even though it’s only 3 years old).

 

[Dyorsolutions]

Primary side image

 

[Dyorsolutions]

Secondary side image

 

[waross]

You don't have access to the primary currents.
Actually balancing the currents on the delta side by moving loads on the secondary side is more difficult than you realize.
When you remove or add a load to a secondary phase, the current changes on two primary phases.
I had a manager who became worried about unbalanced currents on our generators and sent the crew out to switch single phase transformers from phase to phase to try to balance the load on the generators.
He didn't realize that some of our customers had AC and many didn't.
The load balance tended to change as the temperature rose.
Balanced loads at 8 AM were unlikely to be balanced at 3 PM.
I sat back and watched the crew confuse themselves.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dyorsolutions]

@waross

Oh I agree 100%

I had someone say we’d move equipment around to balance the load and my answer was immediately, “no!”

To me that’s a never ending objective with constant variable change. (As you said)

However a statement you made peaks a strong question or two.

At any given time our demand for amperage per phase can be 270amps, so a normal reading for example could be 235a/235a/235a per phase on each 400a breaker (which we have 3 breakers per panel; this xfrmr powering 4 units), that’s in a well running unit with even load.

Main issue I’m seeing is numerous equipment will shutdown and no longer pull their peak amperage demand.
So where:
phase1-breaker1 is pulling 90amps
Phase1-breaker2 is pulling 215amps
Phase1-breaker3 we’ll say is 135amps

1.) Could that alone, (amp imbalance), be causing the transformer to act up to the point it climbs up the pole and blows the fuse?

Or

2.) would this more than likely point to a bad winding in the transformer and needs to be repaired etc..?

Because at this point we are staging for replacement next week if we can’t determine a root cause other than bad winding(s)
-seeing as 2 other identical setups are not showing/experiencing any faults/issues.

(To be clear we will not be attempting to move equipment to balance secondary load as each section has the same potential demand for amperage, we will be work on optimizing the environment in which equipment resides in order to “balance” the conditions).

 

[waross]

Two competing effects.
If a transformer voltage is low, motors will draw very unbalanced current.
eg:A 3% voltage unbalance may cause a 10% current unbalance. It could be worse.

A failing transformer may have a lower voltage, but that is often a shorted turn and a shorted turn will quickly lead to total failure.

10800V to 416 Volts = 26:1
That should be a current ratio of 1:26
140 Amps on the primary should support 3640 amps on the secondary.
Time to roll the connections.
See if the problem follows the secondary loads or stays with the same transformer.
One other possible issue;
Is the fuse holder and the cutout clean?
A bad or corroded connection either to the fuse or to the line conductors will generate enough heat to blow a fuse.
A new cutout, properly installed, may be the quickest trouble shooting course of action to check a cutout.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

On HV lines rated 12470kv we have 3 primary wires coming down via individual conduit;

Another possible problem area.
Are the conduits steel or magnetic?
140 Amps is enough to cause conduit heating due to magnetic encirclement.
The hot conduit will heat the conductor.
I saw a case where a current of about 100 Amps competely destroyed a 400 Amp switch from heat corrosion.
I was told that the same thing had happened at least two times before.
You could not hold your hand on the conduit due to the temperature.
Check for hot conduits.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

1) Having full load on all three phase is current through the primary fuse than have full load on one phase and less than full load on other phases of a delta/wye transformer. (Assuming all loads have similar power factor.)
2)I have not yet imagined what kind of winding failure could result in a fuse blowing intermittently. If there was a problem with the transformer winding, it seems like the fuse would have immediately blown on energization.

Having to tap the transformer 15% from 12470 V to 10800 V seems like an indication this site is way outside the norm for my area. Is the plant power factor very low? Or a mix of some machines with good power factor and some with poor power factor? As a utility engineer, I get very nervous when I hear customers have "fixed" their low voltage issue by adjusting taps. I would much rather hear that customers are having a voltage issue, and find a way for the utility to get service voltage back within the allowable ANSI voltage window.

Do you have voltage monitoring from when the fuse blew? If the utility voltage had been running somewhat above 12.47 kV, the 10.8 kV transformer winding would have been extremely overvoltaged.

With unbalanced loads it makes sense to me that the same fuse could blow every time assuming the same phase was always the heaviest loaded. Having the utility take current/voltage reading makes sense to me.

While achieving perfect balance is not practical, putting all load on a single phase is also not practical. For 12.47 kV circuits, an imbalance of 100 amps is enough to cause a problematic voltage unbalance for motors. Large current imbalances also increase the likelihood of the substation ground element miscoordinating with fuses during line-to-ground faults.

 

[Dyorsolutions]

@waross

Quick answer to your follow up post, we use plastic conduit from riser to ground, i thermal gun those and all seem well.

To your initial reply (which I extremely appreciate this knowledge as I also get to learn):

<terminology/reply clarification>:

-“time to roll the connection”?

-“3640amps on secondary”, per secondary phase or all 3 phases combined?

—————


This week (Thursday or Friday, due to weather), I’ll have utility come inspect cutout holder and fuse barrel etc… (excellent recommendation)!


Also to step back just a bit (if, time to roll the connections means put full load on and see what happens):
Then the only difference as of current moment that’s been done is a 200amp fuse in place of the previous 100amp, only on middle cutout not all of them. Each time we put on full load - cutout fuse blows in approx 1hr, twice now.

If roll the connections means set tap back to rated HV “A” 12470 @ 415/240v and see where the problem goes then our equipment might not run properly if it can’t step up to voltage.

(These questions/statement are my lack of knowledge, clearly not yours.)

I’m ready to test this transformer with troubleshooting in any direction because we’re going to have to replace it next week if it’s not cured, so just seeking clarification for a very motivated but novice person as myself.

I’ve followed other threads you’ve replied to and respect your input greatly, and appreciate angles of input from others as well.


(Even if we replace before resolve, I’m having xfrmr tested and sampled for root cause and will update this thread with whatever results come of it).

 

[waross]

Roll the phases:
A phase conductors moved to B phase.
B phase conductors moved to C phase.
C phase conductors moved to A phase.

Bacon4life:
I have had an exciter winding that tested perfectly when cold, but that would short out when hot.
The issue may be heat related. That would correlate with the fuse blowing under full load.

My feeling is that either the transformer or the cutout are suspect.
Rolling the primary conductors, so that the problem cutout is feeding a different transformer may be done either at the cutouts or at the transformer.
With the problem being in the cutout, the same cutout will blow even when it is feeding a different transformer.
With a transformer problem, the cutout feeding that transformer will blow.
The first step in problem solving is to determine;
What is the problem?
Trying to fix a transformer problem when the issue is actually a cutout will be frustrating, confusing and expensive.
So, First step;
Is the problem the cutout or the transformer.
I am not always right, but I hope that I can help you.

Rolling the primaries versus changing the cutout:
This is mostly a utility decision, and safety is involved.
Changing a cutout requires either hot line work or an outage on the entire circuit.
Depending on local safety rules, rolling the phases on the load side of the cutout may not require an outage.
If rolling the phases shows a bad cutout, the utility will do what they have to do.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Dyorsolutions]

@bacon4life & @waross

You both just gave me some good knowledge to go with,


@bacon4life
We don’t necessarily “choose” whether to put everything on a single phase or not, rather all possible slots are filled and whether the equipment operates properly or not, dictates how much amperage demand hits that ‘said’ phase. (All loads have equal potential, or at least we try to make the environment equal and see how the equipment operates)

But yes load on secondary wye is only possible due to primary delta ability; very true.

@waross
Winter/summer:
We absolutely see increase in issues during summer weather vs. almost none during winter (logic would lean toward hot transformers like to be cool) that being said we currently rely on ambient temperatures & weather for cooling alone but are looking to incorporate fans.

@waross & @bacon4life
1:26 ratio equation is a good point to state (amperage should be handled well)
along with the fact that we are contracted for 12470kva but having to “correct the contract” by tapping to a 10800kva to make up for a “lack of voltage delivery”. Xfrmr tap “A” is rated 12470kva secondary 415/240v (why can’t we receive what’s contracted and not correct with taps as you stated)
However I still get puzzled on how out of 8 transformers only this one is having an issue (I would think is it’s reacting poorly to HV line then the others would be too?)


This week I will attempt:

- roll the phases on primary
- fuse all cutouts at 100amp (for control testing)
- Introduce full load (all 4 units)
- Voltage read HV line


————-

-We do not have live or recorded readings of events

-if transformer blows a different cutout fuse after roll we will lean to replace transformer and I will update with results and testing from lab

-if same cutout fuse blows I will request top of pole have maintenance done

 

[FacEngrPE]

is the delivery voltage 10.8 kV all of the time, or does it increase when the utility load (from other customers) decreases? Monitoring for a week with a 3 phase power quality monitor may be worthwhile.

 

[waross]

Monitoring for a week with a 3 phase power quality monitor may be worthwhile.

The modern revenue KWHr meters have the ability to save several months of information at 15 minute intervals.
One utility that I worked with, I could drop into the engineering office with a couple of 2 liter bottles of Pepsi and a floppy disk.
In a couple of days I could pick up my floppy with a couple of months of data from the revenue meter.
Note: The meters had to be pre-programmed to save the data. Not all meters in the system were programmed to save data.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!